The hydrostatic pressure on the tissue side of the endothe-lial pores is the tissue hydrostatic pressure. This pressure is determined by the water volume in the interstitial space and tissue distensibility. Tissue hydrostatic pressure can be increased by external compression, such as with support stockings, or by internal compression, such as in a muscle during contraction. The tissue hydrostatic pressure in various tissues during resting conditions is a matter of debate. Tissue pressure is probably slightly below atmospheric pressure (negative) to slightly positive (< + 3 mm Hg) during normal hydration of the interstitial space and becomes positive when excess water is in the interstitial space. Tis sue hydrostatic pressure is a filtration force when negative and an absorption force when positive.
Support stockings are routinely prescribed for people whose feet and lower legs swell during prolonged standing. Standing causes high capillary hydrostatic pressures from gravitational effects on blood in the arterial and venous vessels and results in excessive filtration. Support stockings compress the interstitial environment to raise hydrostatic tissue pressure and compress superficial veins, which helps lower venous pressure and, thereby, capillary pressure.
If water is removed from the interstitial space, the hydrostatic pressure becomes very negative and opposes further fluid loss (Fig. 16.6). If a substantial amount of water is added to the interstitial space, the tissue hydrostatic pressure is increased. However, a margin of safety exists over a wide range of tissue fluid volumes (see Fig. 16.6), and excessive tissue hydration or dehydration is avoided. If the tissue volume exceeds a certain range, swelling or edema occurs. In extreme situations, the tissue swells with fluid to the point that pressure dramatically increases and strongly opposes capillary filtration. The ability of tissues to allow substantial changes in interstitial volume with only small changes in pressure indicates that the interstitial space is distensible. As a general rule, about 500 to 1,000 mL of fluid can be withdrawn from the interstitial space of the entire body to help replace water losses due to sweating, diarrhea, vomiting, or blood loss.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.